1. Field of the Invention
The present invention relates to duplexers suitable for use in high frequency bands such as a microwave band, and the invention also relates to communication apparatuses incorporating the same.
2. Description of the Related Art
Recently, with a rapid expansion in miniaturization of mobile communication apparatuses such as cellular phones, there has been a growing demand for miniaturization of duplexers incorporated in the mobile communication apparatuses.
A conventional type of duplexer uses variable frequency filters as a transmission filter and a reception filter. In this case, each of the variable frequency filters has a structure in which dielectric resonators are connected to variable impedance elements such as PIN diodes via reactance elements such as capacitors. With this structure, resonant frequencies can be varied by controlling voltages applied to the variable impedance elements. In addition, in the duplexer incorporating the variable frequency filters, the size of the duplexer is reduced by decreasing the number of the dielectric resonators of the transmission filter and the reception filter.
However, when variable frequency filters are used both as the transmission filter and the reception filter in the duplexer, although miniaturization of the duplexer can be achieved to some extent, there is still a limit to the miniaturization, since dielectric resonators are used both in the transmission filter and the reception filter. Specifically, since the dielectric resonators need to have certain lengths for use at the intended frequencies, it is difficult to miniaturize the duplexer.
Meanwhile, in duplexers disclosed in Japanese Unexamined Patent Application Publication No. 5-95204 and Japanese Unexamined Patent Application Publication No. 5-175879, dielectric filters are used in the transmission filters and surface acoustic wave filters are used in the reception filters. In such duplexers, the surface acoustic wave filters used in the reception filters are smaller than dielectric filters. Thus, the sizes of the duplexers are reduced.
By using surface acoustic wave filters as both the reception filters and the transmission filters, the duplexers could be made smaller. However, usually, since a surface acoustic wave filter has a power rating lower than that of a dielectric filter, it would be difficult to use such a surface acoustic wave filter as a transmission filter, which receives a large amount of electrical power.
In addition, in the Japanese cdma-One cellular phone system, transmission frequencies are allocated in a range of 887 to 925 MHZ, and reception frequencies are allocated in a range of 832 to 870 MHZ. That is, the width of each of the transmission and reception frequency bands is 38 MHZ, which is relatively broad, and the separation bandwidth between the transmission side and the reception side frequencies is 17 MHZ, which is relatively narrow.
In the duplexers of Japanese Unexamined Patent Application Publication No. 5-95204 and Japanese Unexamined Patent Application Publication No. 5-175879, the surface acoustic wave filters are used in the reception filters, and can be miniaturized to some extent. However, due to characteristics of the surface acoustic wave filters, it is difficult to apply the surface acoustic wave filters in broad band applications such as the cdma-One system. Furthermore, even if the bandwidth of the surface acoustic wave filter could be broadened, it would be difficult to obtain a large amount of attenuation, and it would also be difficult to perform phase synthesis with the transmission filter, that is, it would be difficult to provide zero impedance in the transmission frequency band. As a result, particularly, transmission characteristics would be deteriorated.
Accordingly, it is an object of the present invention to provide a compact duplexer suitable for use in communication systems of broad bandwidths with narrow separations, while having good characteristics. It is another object of the present invention to provide a communication apparatus incorporating the duplexer.
According to a first aspect of the present invention, there is provided a duplexer including a first filter having a variable frequency filter formed by connecting a variable impedance element to a dielectric resonator, and a second filter having a plurality of surface acoustic wave filters with mutually different frequency pass bands and a switching circuit for selecting one of the plurality of surface acoustic wave filters.
In this duplexer, one of a plurality of frequency bands can be selected, in the first filter, by controlling the variable impedance element; and in the second filter, by selecting one of the surface acoustic wave filters having different pass bands via the switching circuit.
In this case, the switching circuit may be connected either to the inputs of the surface acoustic wave filters or to both the inputs and outputs thereof.
With the above arrangement, each of the first and second filters can be adapted to the frequency of the counterpart filter by switching the frequency passbands of the first and second filters, respectively. In other words, in effect, the transmission frequency band and the reception frequency band are both narrowed and the separation between both bands is broadened. Thus, narrow band surface acoustic wave filters can be used. As a result, a duplexer having a sufficient amount of attenuation and satisfactory characteristics can be obtained.
Furthermore, by using the switching circuit to select one of the plurality of surface acoustic wave filters, the transmission and reception frequency bands are both narrowed, whereby phase synthesis with the counterpart filter can be easily and satisfactorily performed. In addition, since deterioration of transmission characteristics and reception characteristics due to mismatching of phase synthesis can be prevented, satisfactory transmission/reception characteristics can be obtained.
That is, since the variable frequency filter is used in the first filter and surface acoustic wave filters are used in the second filter, miniaturization of the duplexer can be achieved and characteristics suitable for use in the communication system with broad band width and narrow separation can be obtained.
The plurality of surface acoustic wave filters used in the duplexer according to the present invention include not only surface acoustic wave filters with entirely different frequency pass bands but also surface acoustic wave filters with frequency pass bands that partially overlap within the overall range of the frequency pass bands.
In addition, generally, it is preferable to use a filter including dielectric resonators, which has a high power rating, as the transmission filter to which is applied high power, and a filter including surface acoustic wave filters, which has a low power rating, as the reception filter which receives low-power signals.
In addition, the first filter may be formed as a band elimination filter, and the second filter may be formed as a band pass filter. A PIN diode may be used as the variable impedance element of the first filter, and a switching circuit formed by a PIN diode or a GaAs switch may be used as the switching circuit of the second filter.
Furthermore, the first and second filters may be disposed together on the same substrate to be integrated with each other. As a result, easy handling can be achieved, and stable and satisfactory characteristics can be obtained.
According to a second aspect of the present invention, there is provided a communication apparatus including the dielectric filter or the duplexer having the characteristics described above. Thus, the communication apparatus can be compact and can have satisfactory characteristics.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.